At the time of the present invention, integrated circuits (ICs) in a wide variety of sizes and for a wide variety of purposes have been developed and commercialized. ICs are typically rectangularly shaped dies, also called chips, of semiconducting silicon upon which solid state transistors and interconnecting circuitry have been constructed, and connection to off-chip circuitry is provided through contact leads and bonding pads arranged around a periphery of such a chip.
To protect an IC die and to provide a standard physical interface to, for example, a printed circuit board, such die are typically encapsulated in plastic or ceramic material with wires bonded to bonding pads on a die and to conductive leads leading away from the die. The conductive leads are ultimately soldered to electrical traces on a printed circuit board for use in any one of many electronic applications.
It has been a trend in the art to accomplish increased device density and smaller circuitry dimensions in ICs, to provide more circuitry in a smaller space. There is a trend as well to accomplish increased speed at which an IC operates. As ICs become more complex and powerful as a result, thorough testing of many thousands (sometimes millions) of transistors and miles of interconnective traces has become ever more critical. Typically, an effort is made to test every transistor and every connection of an IC to assure proper operation.
The present invention is in the area of test apparatus for testing ICs. In following descriptions and discussions, ICs to be tested and under test will be referred to generally as device-under-test (DUT), as is common in the art.
Partly because of a trend to smaller die and to more contacts to the outside world from an IC, reliably connecting bonding pads on a die to perform thorough testing is not a simple exercise. Because of this, most ICs are tested after packaging, although in some cases ICs are tested before packaging, and even before individual ICs are separated from semiconductor wafers upon which they are constructed. Methods are also being developed to mount ICs on circuit boards without packaging.
The IC testing process is historically one of placing a packaged IC in a test socket, with leads of the IC making electrical contact with contact pads or lands in a socket. In present art, a computerized tester, to be more fully described below, comprising a CPU, power supply, suitable memory, and control routines developed for testing an IC, provides test vectors, power, and ground connections through an interface harness to a test head. In a test head, power, ground, and vector signal lines are interfaced through a somewhat complex distribution system, which includes pin electronics boards, and provided finally to leads of a DUT placed in a test socket connected to a load board.
Typically, each individual DUT has a dedicated set of software control routines to be executed on a tester to provide a serial stream of test vectors for a DUT. There will also typically be an especially designed and constructed load board for a unique type of DUT, and a specific socket interfaced to a load board, and a particular power and ground connection to suit the unique configuration of the DUT.
In design and development of an IC, information is also used to develop software for testing ICs manufactured according to a particular design. A customized load board and a customized socket is also typically developed. One may have, for example, a batch of 100 packaged ICs of a particular design to be tested. To prepare to test these devices, a customized load board is assembled to a test head. A test socket, with connections configured to meet appropriate leads on a DUT, is assembled to the board, and customized software for that DUT is loaded into a test system.
After a test apparatus is customized to a DUT and a dedicated software is loaded, the 100 DUT's are installed and removed serially, with a test vector set applied to each in order. Typically, if a DUT fails, a test system reports the failure. After the 100 devices are tested, much of a test system (tester, software, load board, socket) is reconfigured for a new batch of DUT types to be tested. Test systems are sometimes manual, and sometimes complex material handling systems may be interfaced to a test system for presenting DUTs to a tester.
The shape and size of IC packages varies, the number of leads varies from type to type, the size and geometry of leads varies, the position of leads where the power supply voltages and grounds must be applied differs from type to type, and so on. Different standard packages are given names, such as dual in-line package (DIP) and quad flat package (QFP). In current art, load boards are designed and manufactured to fit a unique configuration of each DUT.
In many cases, although a single load board may be able to serve a variety of different DUTs having a common package configuration and lead geometry and configuration, power connections and ground lead locations will vary. Load boards, in these cases, have to be configured somehow so power supply and ground connections match with connective requirements of a unique DUT.
In some instances, user's prefer a completely customized board, so power and ground connections terminate at points aligning with socket connections. This is a relatively expensive solution, requiring an entirely different, fully customized board for each DUT, even if several devices to be tested have a common size and lead configuration and can be interfaced to a common socket.
An alternative solution is to bring signal leads to all lead positions at an interface area on a load board, and to provide power and ground points or pads nearby each lead position. Then power and ground may be jumpered to appropriate points at the interface according to the unique requirements for each DUT. In this manner, a common load board may be provided for a large number of DUTs, and customization is done on the common board by one or another finishing technique.
One method of finishing and customizing a load board of the type described above, involves hand-soldering a solder bridge or a jumper wire from power and ground locations nearby, to pads contiguous with through-holes to which socket or receptacle pins may be mounted. That is, given a pattern, for example, of through-holes on a load board to which pins of a receptacle or a socket will be engaged, the through-holes typically corresponding to the leads of a DUT, one simply provides a jumper one-at-a-time to each through-hole for a pin requiring power or ground.
There are some problems inherent in soldering on such a load board, however. Plated through-holes and solder pads on a load board are typically very small and necessarily closely-spaced. This makes such a soldering process, either with or without jumper wires, time-consuming, costly, and somewhat risky. Such solder connections must be very carefully made on a board that may represent a several thousand dollar investment, without damaging plated through holes, contact pads, or surface of the load board. An improper connection or damaged test surface may mean a scrapped board and costly delays.
A new apparatus and method for customizing load boards quickly, inexpensively, and without risk of loss and delay is clearly needed.
For those leads of a DUT that require power, it is conventional in the art that a power connection be made to each such lead, and, during testing each of these leads is typically hot. Still, during testing, depending on the test vectors provided, the power draw on each such lead varies. At various times a relatively sudden demand for increased power may be made on one lead or another, or on all power leads at the same time.
When a demand for power is relatively sudden, due to multiple junctures and often necessarily small cross-section of conductive paths for power from a power supply to final connection to leads of a DUT, it has become a practice to provide decoupling capacitors near each power lead connection, with one end of the capacitor connected as close as is practical to the power lead, and the other connected to ground. These decoupling capacitors are typically and conventionally mounted between plated through holes and a ground plane on the backside of a load board; that is, the side opposite the side where a receptacle and a test socket are mounted.
To provide the best possible decoupling for power leads, closer coupling of capacitors to power leads is needed.